1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for a lead-in cable having a replaceable portion or a lead-in additional cable configured to be mounted between a lead-in cable and a streamer.
2. Discussion of the Background
Oil companies frequently use images obtained by seismic and/or electromagnetic exploration of underground geological structures to select sites for drilling wells.
A marine survey system, as illustrated in FIG. 1, includes a vessel 10 towing a source array 20 and streamers 30 (only one side relative to sail-line S is shown) in direction T. Vessel 10 pulls streamers 30 via lead-in cables 35 (only two labeled in FIG. 1), which are strength members able to convey the necessary towing force. The lead-in cables customary house functional cables such as an electric cable to transmit power from the towing vessel to the streamers. Streamers 30 are usually towed to have parallel trajectories at distances d in the range of 50 and 150 m there-between. In order to achieve this arrangement, one or more deflectors or paravanes such as 37A and 37B (connected to vessel 10 via wide tow lines 39A and 39B and to the outermost lead-in cable via a spur line 40) pull the streamers laterally relative to the towing direction T. Separation ropes 33 (only two labeled in FIG. 1) that are connected between lead-in cables 35 at locations 31 (close to the streamer 30's upstream end) prevent distances between adjacent streamers from exceeding the distance d. The lead-in cables, which have a length in the range of 1000 to 1500 m (depending how far they reach laterally relative to the sail-line S), are prone to damage in portions where the separation ropes are attached.
There are other portions of the lead-in cables which are prone to damage. For example, as illustrated in FIG. 2, lead-in cable 35 is usually heavy and may sag at a depth h below a desired depth of streamer's 30 head (the ideal shape is illustrated as a horizontal dashed line in FIG. 2). A float 36 may be connected to lead-in cable 35 via cable 34 at a location 38 (which is close to the streamer 30's head) to lift the lead-in cable at an intended depth. The portions surrounding locations 31 and 38 (where separation rope(s) 33 and cable 34 are attached to lead-in cable 35) are more prone to damage due to the additional lateral forces (i.e., forces that are not along the lead-in cable). To alleviate this problem, these portions are reinforced using bend restrictors.
In addition to illustrating the portions more prone to damage than the rest of the lead-in cable, FIG. 2 also illustrates the manner of acquiring information regarding the geological structure under seafloor 42. A wave 22 (e.g., a seismic or an electro-magnetic wave) generated by source array 20 penetrates seafloor 42 and is at least partially reflected at an interface 44 between a layer 43 inside which wave propagates with a first velocity and a layer 45 inside which wave 22 propagates with a second velocity different from the first velocity. Reflected wave 24 is then detected by detectors carried by streamer 30. Note that streamer 30 maintains an intended depth profile (which is shown parallel to water surface 15, but may also be a curved depth-varying profile) using position controlling devices 32A and 32B (e.g., birds).
Inspecting and, if necessary, repairing the damaged portions of lead-in cables during at sea (i.e., onboard the vessel) takes a relatively long time. Since onboard time is very expensive, it is desirable to find methods to reduce maintenance time related to the lead-in cables and to prolong usage period thereof.